2.8 explain the relative reactivities of the elements in Group 1 in terms of distance between the outer electrons and the nucleus

Reactivity of Group 1 metals

The metals in Group 1 increase in reactivity as you descend down the group. This is because of the distance between the outer electrons and the nucleus. 

In Group 1 all the atoms have 1 outer electrons. They don't want this electron so are trying to get rid of it to get a full outer shell. 

If the distance between the nucleus and the electron is small then there is a strong force of attraction so it's hard for it to give away its electron.

Similarly if the distance between the nucleus and the electron is great then there is a weaker force so it can give away its electron easily, thus being able to react easier with other atoms. It's more reactive.